The long term goal of the project is to provide clinicians with scientific information which they could use to assist in making the best selection of which restorative material they would incorporate in providing treatment options for patients. Presently, there is an increase in usage of composite resins as posterior restorations. To date most research efforts have been devoted to improving the wear resistance of composite resin restorations. Little effort has been made to determine possible wear of the cusp opposing the composite restoration. The specific aim of this proposed study is to evaluate the effect of a series of five composite in lay systems, of varying composition and particle size, on the wear resistance of human enamel. This project will consist of two phases. Phase I will consist of an in vitro study and Phase II an in vivo study. In Phase I caries free extracted human molars are selected for the study. The specimen is prepared and an composite in lay fabricated and cemented. The specimen is mounted and inserted into a wear test apparatus. This wear device can be used for the critical evaluation of all restorative materials and in fact has been used in conjunction with past studies as well as implants. The teeth selected as the antagonist will consist of the buccal cusp of mandibular first premolars. Small acrylic resin beads are used to serve as the food bolus thereby generating as well as the restored surface of the molar will be evaluated using an image-analyzing system. The surface area for wear facets is calculated automatically by a computer at a magnification of x5O. The values of the surface area for wear facets will be statistically analyzed by a one-factor analysis of variance (P-1 <05). Multiple comparison among respective composite resins will be made with the Fisher test at a 95% level of confidence. In Phase II one hundred composite inlays will be inserted into Class I and II cavity preparations in vivo. All restorations will be inserted over a ninety day period by two clinicians following a strict clinical protocol. The antagonist cusp and the posterior restoration will be evaluated at baseline 3 mos., 6 mos., 12 mos., 24 mos., 36 mos. and 48 mos. By indirect evaluation using epoxy resin models, the wear of the opposing cusp as well as the composite restoration will be determined using an image analyzing system. The same protocol for analysis of data used in Phase l will be repeated for Phase Il.